Magnetic-based torque/speed sensor

ABSTRACT

Torque is measured in the chain wheel ( 50 ) of a pedal cycle by a non-contacting magnetic-based transducer. A magnetising source ( 70 ), a D.C. type of source such as a permanent magnet is positioned adjacent the chain wheel ( 50 ) to induce an arcuate magnetised zone ( 72 ) in the wheel ( 50 ) as it rotates. The zone emanates a torque-dependent magnetic-field component that is detectable by a sensor ( 74 ) that follows the source ( 70 ) in the direction of rotation. By having the magnetising source ( 70 ) continually in position the arcuate tranducer zone ( 72 ) is refreshed on each rotation of the chain wheel. The nature of the pulsating torque in the chain wheel due to the exertion of the rider and its relationship to the angle of the pedal cranks is discussed with a preference for positioning the source ( 70 ) and sensor ( 74 ) at positions of minimum and maximum exerted torque respectively. If the induced arcuate magnetisation is interrupted ( 76 ) pulses for measuring rotational speed are obtainable. The torque measurement is of general application to torque in sprocket wheels or gear wheels.

FIELD OF THE INVENTION

This invention relates to a torque sensor arrangement in which torque istransmitted between two parts rotatable about an axis. The parts may belocated at the same radius and mechanically coupled together, and moregenerally may be two axially spaced parts at the same or differentradii. Of more immediate interest are parts located at different radiiand radially coupled for the transmission of torque from one to theother. Of particular interest is where the parts lie essentially in aplane such as in a disc form of structure, for example a sprocket wheel.

The invention also relates to a rotational speed sensor arrangementwhich incorporable as part of a torque sensor arrangement but which canbe independently applied without being directly involved in torquetransmission.

BACKGROUND TO THE INVENTION

A magnetically-based torque sensor arrangement for a disc-like structureis described in U.S. Pat. No. 4,697,460 (Sugiyama et al). This patentdiscloses an arrangement for torque measurement in an automobiletransmission. An energiser coil/detection coil assembly isnon-contactingly placed adjacent a disc in which torque stress occurs.The energizing coil is A.C. energised to establish an alternatingmagnetic flux in a flux path through the disc that is torque sensitive.The detection coil senses torque-dependent changes in the circulatingmagnetic flux.

Another magnetically-based torque sensor arrangement for a disc-likestructure such as a sprocket wheel is disclosed in PCT applicationWO01/13082 published 22^(nd) Feb., 2001. This application disclosesstructures in which a permanent or stored magnetisation is associatedwith the disc to emanate a torque-dependent magnetic field component.The annular magnetisations employed for this purpose are furtherdiscussed below with reference to FIGS. 1-5.

A contrast can be drawn between using the disc structure as a medium forthe A.C. coupling of an excitation coil to a detector and the use ofstored or permanent magnetisation which itself can be regarded more as aD.C. type of magnetisation. This may be referred to herein as D.C.magnetisation.

The invention has particular, though not exclusive application, totorque transmission arrangements which, in operation, transmit torque ina pulsating or cyclic manner such as is found in the transmission oftorque from the pedal cranks of a pedal cycle to the chain-engagingteeth of the pedal sprocket wheel, usually referred to as the chainwheel. In this case the torque is transmitted between an inner hubportion of the chain wheel and an outer annular tooth-bearing portion.The drive provided by the leg motion of the cyclist tends to approachzero when the pedal cranks are vertical, assuming the force exerted bythe driving foot is vertically down. Without going into details ofcycling motion and mechanics, it has been found that the maximum torqueis usually exerted on the descending pedal crank in an arc extending toeach side of the horizontal, though the arc may tend to advance (ascend)depending on the manner in which the cycle is being ridden. Thegeneration of torque by the cyclist is made more complex because therotational mounting of the pedals itself provides another free axis. Therider can angle the pedals to obtain the maximum mechanical input.Additionally the position of the rider is relevant, e.g. as betweennormal upright riding and a racing position. This maximum torque phaseoccurs twice for each revolution of the pedal cranks, once as each crankreaches the maximum transmission arc. This may be referred to as themaximum torque phase.

Heretofore it has not been possible to reliably and economicallyascertain the torque transmitted in a chain wheel while the cycle isbeing ridden in normal use, and particularly the torque exerted in themaximum torque phase. More particularly it is desirable to be able tomake such a measurement under normal cycling use without inconveniencingthe cyclist.

The practice of the present invention provides a magnetically-based,non-contacting form of torque sensor arrangement having a magnetisedtransducer region which provides a torque-dependent magnetic fieldcomponent and a non-contacting magnetic sensor. The preferred practiceprovides for automatic refreshment or compensation of thetorque-dependent magnetic field which is to be sensed. The invention maybe implemented to provide rotational speed measurement as well as torquesensing. The speed measurement may be implemented of its own right. Theinvention has particular application to cranks that are manually drivenby hand (arm) or foot (leg) in which the torque generated is generallynot uniform throughout the rotation of the crank and may also be appliedin engine driven systems where the torque applied to a rotary shaft isnon-uniform. In particular the applied torque may be pulsating, varyingbetween maximum and minimum torque phases: in the case of a pedal cyclethere are two pulses for each revolution of the chain wheel. Theinvention will be further described in relation to pedal bicycle.

The need for a reliable measurement of torque in normal use has arisenas a means of gauging the effort expended by a cyclist in propelling themachine. It has been proposed to assist the rider of a pedal cycle byproviding a motor drive by way of an electric motor powered by a batterycarried by the cycle. The battery should be as compact and light aspossible consistent with providing the drive energy required forreasonable periods. Raising battery capacity increases weight. To obtainthe best use of battery life, it is desirable that the cycle should notbe propelled solely by the battery energy for extended periods but thatthe battery energy should be called on to supplement the energy of thecyclist. The battery energy is only utilised provided the cyclist isproviding at least a certain threshold propulsion energy. The energysupplied by the rider can be gauged by the torque exerted on the chainwheel driving the cycle chain. The present invention enables us toprovide a solution to the torque measurement problem which can be thenused in controlling the energisation of the electric motor.

SUMMARY OF THE INVENTION

Underlying the present invention is the concept of measuring torque in agear or sprocket wheel, such as a bicycle chain wheel, by maintaining amagnetising source adjacent, but preferably not in contact with, thewheel as it rotates so that an arcuate zone of magnetisation isgenerated—this is a D.C. type of magnetisation in the terms discussedearlier—which zone is sensitive to torque in respect of a magnetic fieldor field component associated with it. This field or field component isdetected by a magnetic field sensor. By having the source continuouslyin place the induced magnetisation in the wheel is refreshed or renewedon each rotation of the wheel. By this means subsequent events otherwisedeleterious to a permanently magnetised transducer zone reliant solelyon its own magnetisation can be overcome.

The concept outlined above finds particular benefit in a wheel, such asa pedal cycle chain wheel, in which the wheel is subject to a pulsatingtorque in each cycle of rotation and where it is desired to measure themaximum torque exerted. To this end it is preferred to effect themagnetisation at a point of low torque in the cycle and to detect thetorque-dependent field at a point of high torque.

By interrupting the induced magnetisation in the wheel aspeed-indicative component is obtainable. This feature can beimplemented on its own or in combination with the torque measurement.

Aspects and features of this invention for which protection is presentlysought are set forth in the claims following this description.

The invention and its practice will be further described by way ofembodiments applied to pedal cycles.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a face view of a sprocket or gear wheel in which two annular,radially-spaced magnetisation zones are established to provide anexternal torque-dependent magnetic field;

FIG. 2 is a partial axial section through the sprocket wheel and alsoshows one magnetisation source for creating the two annular axiallymagnetised zones;

FIG. 3 is a diagrammatic representation of the generation of the sensedtorque-dependent magnetic field;

FIG. 4 a shows a modification of the means for creatirg annularmagnetised zones having a configuration similar to that of FIG. 1 bymagnetising means that acts on one side only of the sprocket wheel;

FIG. 4 b shows another modification for creating a single annularmagnetised zone;

FIG. 4 c shows yet another modification for a stepped profilesprocketwheel;

FIG. 5 is a simplified illustration of the sprocket wheel and the meansfor creating two annular circumferentially magnetised zones;

FIG. 6 is a diagrammatic representation of a pedal sprocket (chain)wheel of a bicycle and the torque generation applied to it;

FIG. 7 a illustrates the torque generation phases in a cycle of rotationin upright riding;

FIG. 7 b illustrates the torque generation phases in a cycle of rotationin racing riding;

FIGS. 8 and 9 illustrate the torque generation phases for differentrider efforts in riding according to FIG. 7 a;

FIGS. 10 a and 10 b are face and in line views of the chain wheelillustrating source and sensor placement;

FIG. 11 shows a modification of the chain wheel to providesynchronisation/calibration; and

FIG. 12 illustrates another chain wheel construction.

DESCRIPTION OF PRIOR PROPOSAL

Before discussing the particular application of the invention to a pedalcycle or a similar application, the following description with referenceto FIGS. 1-4 b explains the arrangement and operation of amagnetic-based, non-contacting torque sensor applied to a sprocket wheelor gear wheel through which torque is transmitted and of which a cyclechain wheel is an example. There will be described a non-contactingtorque sensor having a non-contacting magnetic source and anon-contacting magnetic field sensor.

According to WO01/13082 above-mentioned, a proposal to measure torque ina sprocket wheel is illustrated in FIGS. 1-3. These figures illustratethe sprocket wheel as a solid disc in which annular magnetised zones areprovided to provide a region acting as an annular transducer elementproviding a torque-dependent magnetic field output. The magnetised zonescan be established in various ways. The forms of magnetisationillustrated in FIGS. 1 and 2, 4 and 5 are radially-spacedmagnetisations. They are permanent (remanent) or stored magnetisationsfrom which a D.C. magnetic field is emanated under torque.

Looking first at FIGS. 1 and 2, a sprocket wheel comprises a circulardisc 10, which for present purposes is taken to be a solid disc. Thedisc 10 is centrally mounted on a shaft 20 rotatable about an axis A—A.The disc thus has a pair of faces extending transversely, specificallynormally, of the axis. Around its outer periphery or circumference thedisc has teeth 24 designed to engage a chain or the teeth of anothergear wheel. Rotational drive may be applied from the shaft 20 outwardlythrough the disc to the teeth 24 or vice versa. The transmission ofdrive through the disc generates a torque in the disc whose magnitude isto be sensed. For this purpose a magnetic-based, non-contacting sensoror transducer assembly is employed. The sensor has a transducer region22 comprised of magnetised zones 12 and 14 established in the disc whichprovide an externally detectable torque-dependent magnetic field. Theregion 22 lies between the shaft 20 and the peripheral teeth 24 so thatthe applied torque is transmitted through the transducer region. To thisend the disc is formed of a magnetisable material, at least where thetransducer region is provided. The external field is detected bynon-contacting magnetic field sensing devices which, for example, may beof the saturating core type, Hall device type or magneto-resistive type.It is preferred that directional sensor devices are used that can beoriented with the direction of the field to be sensed.

FIG. 2 shows the provision of a magnet system comprising magnets 16 and18 on opposite sides of the disc to establish the two annular magnetisedzones 12 and 14. Each zone is established as an annulus about the axisA—A, as by rotating the disc between the magnets. Each zone islongitudinally magnetised in that the magnetisation extends in the axialdirection and the two zones 12 and 14 have opposite polarities ofmagnetisation as indicated by the arrows in FIG. 2. The two magnetisedzones provide the transducer region 22 (the magnets 16, 18 beingremoved) which from face 11 of the disc 10, say, appears as shown inFIG. 1. At the surface 11 zone 12 provides an annular pole (N) of onepolarity and zone 14 an annular pole (S) of the opposite polaritybetween which an exterior magnetic flux Mr (FIG. 3) is established tolink the poles. The exterior magnetic field vector is radial at anypoint around the annuli. Under torque the vector is deflected or skewedfrom the radial to a position Mr′ to provide a circumferential ortangential component Ms which extends around the annulus. Ms is afunction of torque and has a zero value at zero torque.

FIG. 1 also shows sensors 28 a-28 d oriented for detecting the torquedependent circumferential component M_(s) and sensors 26 a-26 d orientedfor detecting the radial component Mr used as a reference. The sensorsare of the saturating core type which have an inherent optimum responsein the direction of the axis of the core. A suitable sensor circuit ofthis type is that described published International Patent Applicationpublication number WO98/52063. Although four sensors are shown fordetecting each of fields Mr and Ms, this is not essential. One sensorfor each field will suffice. However, there is advantage in using atleast one pair of diametrically opposed sensors in which the inductorscan be so connected as to additively respond to the wanted Mr or Mscomponent while acting to cancel external ambient fields such as theEarth's magnetic field.

Referring to FIG. 2, the zones 12 and 14 are magnetised to saturation soas to retain remanent permanent magnetisation when the magnets 16, 18are removed. In FIG. 1 all the field sensing devices are adjacent face11. To enhance the external field available at face 11, the other facecan be provided with a low reluctance annular bridge or keepermagnetically connecting zones 12 and 14 at that face. Anotherpossibility is the modification shown in FIG. 4 a where the disc 10 issubject to a single magnet source at one side, e.g. the magnet 16adjacent face 11 to produce a magnetised zone 23 closed within thematerial of the disc as indicated by the dotted lines with two radiallyspaced annular poles 12′ and 14′ of opposite polarity between which, asin FIG. 3, an external reference radial field Mr exists and atorque-dependent circumferential or tangential field Ms is producedunder torque.

Other modifications are possible. For example, the magnets providing themagnetising source in FIG. 2 could be replaced as indicated in FIG. 4 bby a respective single magnet 16′, 18′ on each side of the disc 10,obliquely disposed with their poles in opposed polarity, so as to createa single annular zone, such as zone 12′ at an angle to the direction oftorque transmission through the disc. An alternative is to rely on aradial offsetting of the magnets to produce an oblique magnetisation inthe disc. Such a zone may also be created by a single magnet pole angledwith respect to the plane of the disc on one side only if the disc isthin enough and the magnet strong enough. A single magnet could also beplaced parallel to the disc surface to have a flux path closed throughthe disc which would magnetise the disc to have two radially spacedpoles, each being an annular zone.

FIG. 4 c illustrates a profiled sprocket wheel having a central hubportion 10 a connected to an outer toothed portion 10 b by anintermediate portion 10 c providing a step between the portions 10 a and10 b which lie in different planes. Similarly to FIG. 4 b a pair ofmagnets 16″ and 18″ are in this case offset to provide a magnetisationzone 12″ at an angle to the direction of torque transmission. It will benoted that the magnets lie within the planes defined by the portions 10a and 10 b.

As will appear below, for pedal cycle use magnetising sources which lieentirely on one side of the chain wheel are more easy to implement asregards not inconveniencing the cyclist. They can be placed adjacent theinside surface.

FIG. 5 illustrates how the torque transmitting sprocket wheel can beadapted to work with circumferential magnetisation. Circumferential(circular) magnetisation is a known phenomenon, though not in the formshown in FIG. 5. Reference may be made to the Garshelis U.S. patentsmentioned below among others. FIG. 5 is a simplified face view of a disc30 through which torque is transmitted between a drive applied on theaxis A and the periphery or vice versa as previously described. In thisembodiment, there is a transducer region 32 which comprises an innerannular region 34 and an outer annular region 36. The regions 34 and 36have opposite polarities of magnetisation as indicated by the respectivearrows. The circumferential magnetisation may be applied through theface 38 using a U-shape magnet arrangement in which the North and Southpoles are aligned normally to a radius as indicated at 40, 42 forannulus 36 rather than being aligned radially as in FIG. 2. Annulus 34is similarly created.

In the absence of torque, the circumferential fields in regions 34 and36 will be trapped within the annular regions. However, under torque thefields become skewed in the manner well-known with prior artcircumferential transducers, e.g. Garshelis U.S. Pat. Nos. 5,351,555,5,520,059 and 5,465,627. The consequence is that at face 38 the regions34 and 36 develop magnetic poles of opposite polarity. The polarity isdependent on the direction of torque.

A radial measurement field Ms is generated externally of the surface 38between regions 34 and 36, the radial magnetic flux being a function oftorque. The radial flux can be sensed by one or more sensors disposed asfor the radial (reference) flux in FIG. 12 a, e.g. sensors 26 a-d. Incontrast to FIG. 1 it is seen that the detectable torque-dependent fluxis radial, rather than circumferential. There is no reference fieldcomponent available.

In describing the magnetising sources for the various remanent or storedmagnetisation configurations, permanent magnets have been shown. Themagnetising source could be realised by appropriately configured D.C.energised electromagnets. However, current magnet technology enables theprovision of compact powerful permanent magnet sources. The latter haveconsiderable advantage in implementing the automatic field refresh orcompensation for a pedal cycle now to be more particularly described.What has been discussed thus far is the use of magnetic sources toinitially magnetise a gear wheel or sprocket wheel, the sources thenbeing removed so that the transducer region depends on the storedmagnetisation. Consideration can now be given to the measurement oftorque in the chain wheel of a cycle and other similarly poweredarrangements.

DESCRIPTION OF PREFERRED EMBODIMENTS

Start with the case that FIGS. 1 and 2 represent a chain wheel. Theshaft 20 is coupled to the usual pair of opposed pedal cranks and thesprocket teeth communicate the rotational drive on the sprocket wheel 10to the cycle chain. If the sprocket wheel is of magnetisable materialthat has been premagnetised as described with reference to FIGS. 1 and 2for example, torque measurement can be made by appropriate placement ofmagnetic field sensor devices. It is assumed here that the cycle has anappropriate source of electrical energy for the torque sensing andsignal processing circuitry. In the context of motor-assisted propulsionas discussed above, there will be an electrical battery source. Sensordevices and the accompanying electronics can be made in very compact,light form and can be placed adjacent the inside surface, i.e. inner,frame-side, surface, of the chain wheel so as to be out of the way ofthe cyclist. The magnets and sensor(s) can be mounted to the frame.

However, potential problems arise in maintaining the stored transducerregion fields in the variety of circumstances under which cycles areused. They may be left propped against metalwork of ferrous metal,possibly in the presence of magnetic fields. There is also a generalproblem of deterioration or leaching of the stored fields over time. Inthe FIG. 1 or FIG. 4 magnetisation, but not in that of FIG. 5, there isa reference field Mr against which the measured torque-dependent fieldMs can be measured. However, the manner in which bicycles are used asjust-mentioned raises the possibility of not only that the stored fieldmay change over time but that the change is non-uniform around theannulus of magnetisation.

To solve this difficulty it is now proposed to provide a magnetic sourceadjacent the chain wheel so that the magnetisation is refreshed on eachrevolution of the wheel.

This solution is then implemented without any pre-magnetisation. Themagnetisation is created and refreshed at a point in the circular pathof the chain wheel and the torque-sensitive magnetic field is read at asubsequent point in the path. The refreshing of the magnetisation isprovided at each revolution of the chain wheel. The magnetisation is ofthe D.C. type as discussed above and the magnetic source employed isalso of this type, that is a permanent magnet or a D.C. energisedelectromagnet. For the application of the invention to pedal cycles,permanent magnet sources will be described. To determine the bestlocations for the placement of the magnetisation source and thesensor(s) for determining the effort expended by the cyclist requires anunderstanding of the mechanics of torque generation and transmission incycling.

FIG. 6 is a diagrammatic illustration of a chain wheel 50 and the usualtwo opposed pedal cranks 52 a, 52 b and their respective pedals 54 a, 54b shown in an arbitrary position in the revolution of the wheel shown byarrows A, the cycle moving forward in the direction of arrow B. Theinput from the rider is essentially from the descending crank 52 a.Torque is generated over an arc 58 about the horizontal axis H—H whichcan be divided into arcs 60 a, 60 b of low to medium torque precedingand succeeding the horizontal axis H—H and an arc 62 of high torqueapproximately centred about the horizontal axis. The arcs shownrepresent a general pattern which can vary from rider to rider and independence upon the manner in which the cycle is being ridden at anytime. The torque pattern is repeated as crank 52 b becomes the leadingcrank.

There are also arcs of minimum torque 64 a, 64 b about the vertical axisV—V (the top and bottom dead centre positions) of the cranks as thechain wheel 50 rotates.

These high, low-medium and low torque phases are summarised in FIG. 7 aand are graphically represented as a function of the rotational positionof the pedal cranks in FIGS. 8 and 9. The angles 90° and 270° are on thehorizontal axis: 0° and 180° are on the vertical axis of FIG. 6.

The rotational torque profile (torque T v. rotational position R_(p))seen by the chain wheel 50 per full 360° revolution will include two“high” torque and two “low” torque phases, denoted “H” and “L”respectively in FIGS. 8 and 9 with about 90° spacing between a hightorque phase and the following low torque phase, and vice versa.Depending on the way the cyclist is using the pedals, the low torquephase may be very close to zero, say less than 10% of the maximum torquevalue. This is illustrated in FIG. 8. However, another cyclist maygenerate a different profile with the low torque phase at about 30% ofthe maximum value as illustrated in FIG. 9.

To illustrate the influence of the manner in which the bicycle isridden, the distribution of the torque phases around the pedal crankcycle is as shown in FIG. 7 a for normal riding in an upright position.However, for riding with greater vigour and particularly when racing,the phases will tend to displace counterclockwise as indicated in FIG. 7b by an angle β up to about 20° though the same pattern of the phasesremains with the high and low phases remaining approximately 90° apart.Thus the torque variation per cycle of rotation remains similar to FIGS.8 and 9 but with the 0° advanced by β from the vertical axis V—V, andthe other rotational positions correspondingly advanced. Thecounterclockwise displacement is again as looking at the outer surfaceof the chain wheel away from the frame.

The object in the present embodiment is to measure the maximum torqueexerted by the cyclist. In turn the measured torque can then be comparedwith a threshold value and the comparison used to control theenergisation of a motor to switch on the motor power to assist the riderwhen the threshold is exceeded. In order to measure the maximum torqueexerted, appropriate placement is required of the magnetic field sensorand of the means for creating and refreshing the magnetic field.

FIGS. 10 a and 10 b illustrate how a magnetising source 70, of whateverspecific form, is placed adjacent but not contacting the inside(frame-side) surface of the chain wheel 50 so that it creates an arc ofmagnetisation 72 around the chain wheel which will extend a full 360° ifthe chain wheel is solid at the radius of the source 70 with respect tothe chain wheel axis of rotation. This arc of magnetisation provides atorque-sensitive transducer zone or region from which a torque-dependentfield component is emanated. The magnetisation is refreshed each timethe chain wheel rotates. The direction of rotation is indicated by arrowR. It is preferred that this magnetisation be effected at a point wherethe chain wheel is not exposed to large torque stresses and at a pointin the chain wheel rotation where the pulsating torque as shown in FIGS.8 and 9 is at a low phase L. A sensor device(s) 74 is position normallyat an angle about the shaft axis that follows the position of the source70 in the direction of rotation so as to be responsive to thetorque-dependent magnetic field emanated by the generated transducerzone 72. In contrast to the source the sensor device(s) 74 is placed ata position in the circular path at the radius of source 70, where thetorque exerted by the rider is at a maximum H. The sensor device is alsoplaced adjacent but not in contact with the inside surface of the chainwheel 50.

Continuing with the solid disc chain wheel, FIG. 11 shows theintroduction of synchronisation/calibration apertures 76 into the discto intersect the magnetised zone 72.

The purpose of apertures 76 is to interrupt the normal measurementprocess of the magnetic field sensor(s) 74 in a regular fashion. Duringinterruption, where the sensor is facing air, the electronic system canreset its zero-point. The dimensions and shape of the apertures are notcritical as long as they are large enough for a clear signal to beobtained during the calibration check but are not so large as to preventa proper signal being obtained during the maximum torque phase. By wayof example, if the sensor device is 8 mm. in diameter, the apertureshould be about 32 mm. The aperture width should be 3 to 4 times that ofthe sensor.

Real chain wheels for pedal bicycles usually have a more open structurewith say spoke portions connecting a central hub portion on the shaft toan outer toothed annulus and through which the torque is transmitted. Ifthe number of spokes is reduced to two, the sort of structure commonlyemployed is that illustrated in FIG. 12 in which a chain wheel 80 has atoothed annular portion 82 having an integral web 84 across it by whichconnection is made to the pedal crank shaft 20. A pair of large opposedapertures 86 are present. The pedal cranks are assumed to be inalignment with the web 84.

The magnetisation source 90 is located adjacent the inner, frame-side,face of the chain wheel 80 at a radius which lies within the outerannular portion 82 so that for much of the revolution of wheel 80 thesource 90 is confronted by space. It is confronted by the web 84 toestablish arcuate magnetised zones 92 a and 92 b at opposite sides ofthe web. Normally the sprocket wheel 80 will be of the same materialthroughout but at least the portions to carry the transducer regionsshould be of magnetisable material or carry magnetisable material towhich torque is transmitted. The source is preferably at a positionabout the axis of crank rotation which corresponds to a minimum torquephase. The sensor or sensors 94 for the torque-dependent field should belocated at an angular position. It will be seen that the signal outputfrom is in the form of pulses coincident with the confrontation of thesensor(s) by the web. There are two pulses per revolution. The pulserate provides a ready means of determining the rotational speed of thepedal sprocket wheel.

The signal output from the sensor(s) 94 can also be sampled in theperiod it is facing an aperture. This can be used to provide acalibration reference related to other ambient parameters which mayaffect the output signal.

As suggested above, the automatic field refresh arrangement requires nopre-magnetisation and therefore no alignment of the magnetising sourcewith a premagnetised transducer region. The described transducerarrangement develops its own magnetisation by the source fixedly mountedon the cycle so as to be closely adjacent the path of the relevantsurface(s) of the sprocket wheel and the sensor is positioned at thesame radius as the source. It may take more than one revolution of thepedal sprocket wheel to fully establish the transducer region(s) at theoutset of motion.

It will be recognised that the speed (pulse rate) sensing can beutilised irrespective of whether torque measurement is being performed.If no torque measurement is required then the positioning requirementsdiscussed above for optimum torque measurement no longer apply.

The processing of the torque measurement signal and relating it to themotor-assisted drive is outside the scope of this invention. Themagnetic field sensor such as that disclosed in abovementionedpublication WO98/52063 can be correlated with torque for any particularillustration. It is worth mentioning that where the torque-dependentsignal is in a form that also provides speed information, such as thepulsed form above discussed, the torque and speed components areindependent. For a cycle the rate of rotation of the pedal sprocketwheel is typically in the range of up to 120 rpm. The amplitude of thepulses represents torque and is independent of the pulse rate.

It is to be understood that while magnetising sources of variousconfigurations have been llustrated in the drawings, otherconfigurations are possible both for planar wheels and for wheels havingother shapes.

1. A torque sensor arrangement for a part rotatable about an axis, thepart having a portion of magnetisable material through which torque istransmitted and which has a surface, comprising a magnetising sourcelocated adjacent the path of said surface as it rotates to generate anarcuate transducer region of stored magnetisation extending inwardlyfrom said surface, said magnetising source being configured to produce atransducer region having an external component of its magnetic fieldthat is a function of torque, and a magnetic field sensor locatedadjacent the path of said surface as it rotates to produce an outputsignal representing said external component.
 2. A torque sensorarrangement as claimed in claim 1 in which said surface extendstransversely of said axis.
 3. A torque sensor arrangement as claimed inclaim 1 in which said part has a disc-like form.
 4. A torque sensorarrangement as claimed in claim 3 in which said part is mounted forrotation about the disc axis and has drive transmitting or receivingmeans at its periphery.
 5. A torque transducer arrangement as claimed inclaim 3 in which said part comprises an outer annular portion connectedto an inner hub portion by one or more spokes.
 6. A torque transducerarrangement as claimed in claim 5 in which said surface is a surface ofsaid one or more spokes.
 7. A torque transducer arrangement as claimedin claim 1 in which said surface extends 360° about said axis.
 8. Atorque transducer arrangement as claimed in claim 1 in which saidsurface extends less than 360° about said axis so as to produce a pulsedoutput from said magnetic field sensor.
 9. A torque sensor arrangementfor a part rotatable about an axis, the part having a portion ofmagnetisable material through which torque is transmitted and which hasa surface, said part having connected thereto means for applyingrotational drive to said part, the drive having a cyclic pattern in eachrevolution of the part that includes an arc over which maximum torque isexerted on the part and an arc over which minimum torque is exerted onthe part, the sensor arrangement comprising: a magnetising sourcelocated adjacent the path of said surface as it rotates to generate anarcuate transducer region of stored magnetisation extending inwardlyfrom said surface, said magnetising source being configured to produce atransducer region extending into said magnetisable portion from saidsurface and providing an external magnetic field component that is afunction of torque; and a magnetic field sensor located adjacent saidsurface as it rotates and spaced apart from said magnetizing source toproduce an output signal representing said external field component. 10.A torque sensor arrangement as claimed in claim 9 in which said magneticfield sensor is located to sense the external magnetic field when saidpart is traversing said arc over which maximum torque is exerted.
 11. Atorque sensor arrangement as claimed in claim 10 in which said magneticfield sensor is located in said arc over which maximum torque isexerted.
 12. A torque sensor arrangement as claimed in claim 9 in whichsaid magnetic source is located to magnetise said portion when said partis traversing said arc over which minimum torque is exerted.
 13. Atorque sensor arrangement as claimed in claim 12 in which said magneticsource is located in said arc over which minimum torque is exerted. 14.A torque sensor arrangement as claimed in claim 9 in which said part isa chain wheel of a cycle and said drive means comprises two opposedpedal cranks and associated pedals.
 15. A torque transducer arrangementas claimed in claim 1 in which said part is a chain wheel of a cycle.16. A torque sensor arrangement as claimed in claim 9 in which saidsurface extends transversely of said axis.
 17. A torque sensorarrangement as claimed in claim 9 in which said magnetising sourcecomprises a permanent magnet assembly or an electromagnet assembly. 18.A torque transducer arrangement as claimed in claim 1, wherein saidtorque transducer arrangement is a speed sensor for measuring therotational speed of said part.
 19. A torque sensor arrangement asclaimed in claim 9, wherein said torque sensor arrangement is a speedsensor for measuring the rotational speed of said part, and wherein saidsurface extends less than 360° about said axis.